Authorities alleged that many of the weapons were bought through “straw purchasers” — people who were simply buying the weapons for the men, not for themselves.

Federal law requires each buyer to complete a Firearms Transaction Record (ATF Form 4473) certifying they are the “actual buyer of the firearm.” The form warns that they are not the actual buyer if they’re buying it for someone else.

“The illegal trafficking of firearms from the United States to Mexico is a serious problem,” U.S. Attorney Sanford C. Coats of Oklahoma said in a statement. “It is fueling a lot of the violent drug cartel activity there.”